Laser apparatus in which GaN-based compound surface-emitting semiconductor element is excited with GaN-based compound semiconductor laser element

ABSTRACT

A laser apparatus includes a semiconductor laser element having a first active layer made of a GaN-based compound, and emitting first laser light; and a surface-emitting semiconductor element having a second active layer made of a GaN-based compound, being excited with the first laser light, and emitting second laser light. In addition, the surface-emitting semiconductor element may have a first mirror arranged on one side of the second active layer, and a second mirror may be arranged outside the surface-emitting semiconductor element so that the first and second mirrors form a resonator.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser apparatus using a semiconductorlaser element.

2. Description of the Related Art

Nakamura et al., “InGaN/GaN/AlGaN-Based Laser Diodes Grown on GaNSubstrates with a Fundamental Transverse Mode,” Japanese Journal ofApplied Physics Part 2 Letters, vol. 37, 1998, pp. L1020 discloses ashort-wavelength semiconductor laser device which emits laser light inthe 410 nm band. This semiconductor laser device is constructed asfollows. First, a GaN substrate is produced by forming a GaN layer on asapphire substrate, forming a GaN layer by selective growth using a SiO₂mask, and removing the sapphire substrate. Next, an n-type GaN bufferlayer, an n-type InGaN crack prevention layer, an AlGaN/n-GaN modulationdoped superlattice cladding layer, an n-type GaN optical waveguidelayer, an undoped InGaN/n-InGaN multiple-quantum-well active layer, ap-type AlGaN carrier block layer, a p-type GaN optical waveguide layer,an AlGaN/p-GaN modulation doped superlattice cladding layer, and ap-type GaN contact layer are formed on the GaN substrate.

However, the output power of the above semiconductor laser device in thefundamental transverse mode is at most about 30 mW. In addition, currentinjection type semiconductor laser devices formed as above deterioratewith elapse of time, due to diffusion of dopants such as magnesium andanticipated short-circuit currents. Therefore, it is difficult toincrease lifetimes of the current injection type semiconductor laserdevices. In particular, when the indium content in the InGaN activelayer is increased in order to obtain laser light of a longer wavelengththan the green wavelength, the characteristics of the crystaldeteriorate, and therefore the lifetime decreases. That is, it isdifficult to obtain high output power from the semiconductor laserdevices having an indium-rich InGaN active layer.

On the other hand, in the conventional semiconductor-laser excitedsolid-state laser apparatuses, it is difficult to achieve high speedmodulation of laser light by directly modulating semiconductor laserelements which are provided as excitation light sources since thelifetimes of fluorescence emitted from rare earth elements constitutingsolid-state laser crystals are very long.

In order to solve the above-mentioned problems, U.S. Pat. Nos. 5,461,637and 5,627,853 propose laser apparatuses in which surface-emittingsemiconductor elements are excited with light. However, since theselaser apparatuses utilize the thermal lens effect, i.e., the effect ofincreasing refractive indexes with temperature, the temperature must beraised. In addition, the above laser apparatuses are sensitive totemperature distribution, and the spatial oscillation mode is unstable.The spatial oscillation mode becomes further unstable when output poweris high, since a cavity is generated in a carrier distribution due togeneration of laser light with high output power (i.e., so-calledspatial hole burning occurs), and refractive indexes decrease withincrease in the number of carriers due to a so-called plasma effect.

Furthermore, CLEO '99 (Conference on Lasers and Electro-Optics, 1999),post-deadline paper CPD15-1 reports a laser apparatus which emits laserlight at the wavelength of 399 nm by exciting an InGaN surface-emittingsemiconductor element with a N₂ laser as an excitation light source atroom temperature. However, this laser apparatus oscillates in a pulsemode having a frequency of 3 Hz, and continuous wave (CW) oscillation isnot realized. In addition, since the N₂ laser is used, the size and costof the laser apparatus are great.

As described above, it is very difficult to realize high-output-poweroscillation in a fundamental mode in the conventional laser apparatuseswhich use a semiconductor laser element, and to emit laser light in thewavelength range from ultraviolet to green.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a reliable laserapparatus which oscillates in a fundamental mode with high output power.

According to the first aspect of the present invention, there isprovided a laser apparatus comprising: a semiconductor laser elementhaving a first active layer made of a GaN-based compound, and emittingfirst laser light; and a surface-emitting semiconductor element having asecond active layer made of a GaN-based compound, being excited with thefirst laser light, and emitting second laser light.

The above surface-emitting semiconductor element may comprise a layeredstructure formed of a plurality of semiconductor layers made of aplurality of GaN-based compounds, and a pair of mirrors may be arrangedon both sides of the layered structure in the direction of the elevationof the semiconductor layers.

According to the second aspect of the present invention, there isprovided a laser apparatus comprising: a semiconductor laser elementhaving a first active layer made of a GaN-based compound, and emittingfirst laser light; a surface-emitting semiconductor element beingexcited with the first laser light, emitting second laser light, andhaving a second active layer made of a GaN-based compound and a firstmirror arranged on one side of the second active layer; and a secondmirror arranged outside the surface-emitting semiconductor element sothat the first and second mirrors form a resonator.

The laser apparatuses according to the first and second aspects of thepresent invention have the following advantages.

-   -   (a) Since, according to the present invention, laser light is        generated by a GaN-based compound surface-emitting semiconductor        element which is excited with excitation laser light emitted by        another GaN-based compound semiconductor laser element, the        semiconductor laser element which emits the excitation laser        light can be a broad area type semiconductor laser element,        which can emit laser light having high output power (e.g., 1 to        10 watts). Therefore, laser light of hundreds of milliwatts to        several watts can be obtained from the laser apparatus according        to the present invention. That is, the laser apparatus according        to the present invention can emit laser light with high output        power in a fundamental transverse mode.    -   (b) Since the thermal conductivities of the GaN-based compound        semiconductor elements are very great (i.e., about 130 W/m·K),        compared with the thermal conductivities of the GaAs-based        compound semiconductors, which are about 45.8 W/m·K, the        aforementioned thermal lens effect is not caused in the        GaN-based compound semiconductor elements. In addition, when an        external mirror is provided, i.e., the aforementioned second        mirror is provided outside the surface-emitting semiconductor        element, laser oscillation can be achieved without using the        thermal lens effect. Therefore, the oscillation mode is stable.    -   (c) Since a semiconductor laser element is used as an excitation        light source, it is possible to realize a laser apparatus which        is highly efficient, inexpensive, and capable of achieving the        continuous wave (CW) oscillation.    -   (d) Since the surface-emitting semiconductor element can be        directly modulated, it is possible to achieve high-speed        modulation of laser light in the wavelength range from        ultraviolet to green.    -   (e) The surface-emitting semiconductor elements used in the        laser apparatuses according to the first and second aspects of        the present invention are excited with light, and are therefore        different from the usual semiconductor laser elements driven by        current injection, in that the light-excited surface-emitting        semiconductor elements are free from the aforementioned problem        of the deterioration with elapse of time due to short-circuit        currents caused by diffusion of dopants such as magnesium. Thus,        the lifetimes of the laser apparatuses according to the first        and second aspects of the present invention are long.

Preferably, the laser apparatuses according to the first and secondaspects of the present invention may also have one or any possiblecombination of the following additional features (i) to (v).

-   -   (i) The first active layer may be made of an InGaN or GaN        material, and the second active layer may be made of an InGaN        material.    -   (ii) The first active layer may be made of an InGaN or GaN        material, and the second active layer may be made of a GaNAs or        InGaNAs material.    -   (iii) The laser apparatus according to the first or second        aspect of the present invention may further comprise at least        one third semiconductor laser element each of which has a third        active layer made of a GaN-based compound, and emits third laser        light, and the surface-emitting semiconductor element may be        excited with the third laser light together with the first laser        light.    -   (iv) The laser apparatus according to the first or second aspect        of the present invention may further comprise at least one third        semiconductor laser element each of which has a third active        layer made of a GaN-based compound, and emits third laser light,        and the surface-emitting semiconductor element may be excited        with fourth laser light which is produced by polarization        coupling of the first and third laser light.    -   (v) The second active layer may include a plurality of quantum        wells. In particular, it is preferable that the number of        quantum wells included in the second active layer is twenty or        more.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a semiconductor laser element whichis used as an excitation light source in a laser apparatus as the firstembodiment of the present invention.

FIG. 2 is a cross-sectional view of a surface-emitting semiconductorelement which is also used in the laser apparatus as the firstembodiment of the present invention.

FIG. 3 is a diagram illustrating the construction of the laser apparatusas the first embodiment of the present invention.

FIG. 4 is a cross-sectional view of a semiconductor laser element whichis used as an excitation light source in a laser apparatus as the secondembodiment of the present invention.

FIG. 5 is a cross-sectional view of a surface-emitting semiconductorelement which is also used in the laser apparatus as the secondembodiment of the present invention.

FIG. 6A is a diagram illustrating the construction of the laserapparatus as the second embodiment of the present invention.

FIG. 6B is a diagram illustrating the construction of the laserapparatus as a variation of the second embodiment of the presentinvention.

FIG. 7 is a cross-sectional view of a surface-emitting semiconductorelement in a laser apparatus as the third embodiment of the presentinvention.

FIG. 8A is a diagram illustrating the construction of the laserapparatus as the third embodiment of the present invention.

FIG. 8B is a diagram illustrating the construction of the laserapparatus as a variation of the third embodiment of the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are explained in detail below withreference to drawings.

First Embodiment

The semiconductor laser element used as an excitation light source inthe first embodiment emits laser light in the 360 nm band as excitationlight. FIG. 1 is a cross-sectional view of the semiconductor laser. Thesemiconductor laser element used as an excitation light source in thefirst embodiment is produced as follows.

Initially, an n-type GaN (0001) substrate 11 is formed in accordancewith the method described in Japanese Journal of Applied Physics Part 2Letters, vol. 37, 1998, pp. L1020. Then, an n-type Ga_(1-z1)Al_(z1)N/GaNsuperlattice cladding layer 12 (0<z1<1), an n-type or i-type (intrinsic)Ga_(1-z2)Al_(z2)N optical waveguide layer 13 (z1>z2>0), aGa_(1-z2)Al_(z2)N (doped with Si)/GaN multiple-quantum-well active layer14, a p-type Ga_(1-z3)Al_(z3)N carrier blocking layer 15 (0.5>z3>z2), ann-type or i-type Ga_(1-z2)Al_(z2)N optical waveguide layer 16 (z1>z2>0),a p-type Ga_(1-z1)Al_(z1)N/GaN superlattice cladding layer 17 (0<z1<1),and a p-type GaN contact layer 18 are formed on the n-type GaN (0001)substrate 11 by organometallic vapor phase epitaxy. Thereafter, a SiO₂insulation film 19 is formed over the p-type GaN contact layer 18, and astripe area of the SiO₂ insulation film 19 having a width of about 100μm is removed by normal lithography. Then, a p electrode 20 is formedover the SiO₂ insulation film 19 and the stripe area of the p-type GaNcontact layer 18, the substrate 11 is polished, and an n electrode 21 isformed on the polished surface of the substrate 11. Finally, a resonatoris formed by cleavage, and a high reflectance coating and a lowreflectance coating are provided on the respective end surfaces so as toform a resonator. Then, the construction of FIG. 1 is formed into achip.

FIG. 2 is a cross-sectional view of a surface-emitting semiconductorelement which is also used in the laser apparatus as the firstembodiment of the present invention. The surface-emitting semiconductorelement of FIG. 2 is excited with excitation laser light emitted fromthe semiconductor laser element of FIG. 1, and oscillates in a singletransverse mode. The surface-emitting semiconductor element used in thefirst embodiment is produced as follows.

Initially, a superlattice distributed reflection film 32, a GaN opticalconfinement layer 33, an In_(x2)Ga_(1-x2)N/In_(x3)Ga_(1-x3)Nmultiple-quantum-well active layer 34 (0<x2<x3<0.5), a GaN opticalconfinement layer 35, and an Al_(z4)Ga_(1-z4)N layer 36 (0<z4<0.5) areformed on a GaN (0001) substrate 31 by organometallic vapor phaseepitaxy, where the superlattice distributed reflection film 32 iscomprised of 20 pairs of AlN and GaN layers, the GaN layer in each pairhas a thickness of λ/4n_(GaN), the AlN layer in each pair has athickness of λ/4n_(AlN), λ is an oscillation wavelength of thesurface-emitting semiconductor element of FIG. 2, and n_(GaN) andn_(AlN) are the refractive indexes of GaN and AlN at the oscillationwavelength λ, respectively. Next, a ZrO₂ antireflection coating 37having a thickness of λ/4n_(ZrO2) is formed over the constructionlayered as above, by electron beam evaporation, where n_(ZrO2) is therefractive index of ZrO₂ at the oscillation wavelength λ. Thereafter,the substrate 31 is polished, and the layered structure formed as aboveis cleaved, and further formed into a chip.

The wavelength λ of light emitted by the surface-emitting semiconductorelement 38 of FIG. 2 can be controlled in the range between 380 and 560nm by appropriately adjusting the composition of the In_(x3)Ga_(1-x3)Nmultiple-quantum-well active layer 34.

In order to sufficiently absorb the excitation laser light, it ispreferable that the number of quantum wells in the multiple-quantum-wellactive layer 34 is 20 or more, and it is further preferable that thenumber of quantum wells is about 24 since the surface-emittingsemiconductor element 38 is prone to crack due to excessive thicknesswhen the number of the quantum wells exceeds 24.

FIG. 3 is a diagram illustrating the construction of the laser apparatusas the first embodiment of the present invention.

The laser apparatus of FIG. 3 comprises the semiconductor laser element24 as the excitation light source, the surface-emitting semiconductorelement 38 bonded to a heatsink 43 at the surface of the substrate 31, aconcave mirror 46 as an output mirror, a resonator 49 formed by aconcave surface of the concave mirror 46 and the superlatticedistributed reflection film 32 of the surface-emitting semiconductorelement 38, and a Brewster plate 45 arranged in the resonator 49. TheBrewster plate 45 controls polarization.

In the construction of FIG. 3, excitation laser light 47 emitted fromthe semiconductor laser element 24 is collected by the lens 42 into thesemiconductor layers of the surface-emitting semiconductor element 38,and excites the surface-emitting semiconductor element 38. Then, lightemitted by the surface-emitting semiconductor element 38 resonates inthe resonator 49, and laser light 48 exits from the output mirror 46.

Since the GaN substrate 31 of the surface-emitting semiconductor element38 is not transparent to the excitation laser light 47 emitted from thesemiconductor laser element 24, the surface-emitting semiconductorelement 38 is excited with the excitation laser light 47 from thelateral side of the surface-emitting semiconductor element 38, asillustrated in FIG. 3.

The laser apparatus of FIG. 3 has the following advantages.

-   -   (a) Since the thermal conductivity of the GaN substrate 31 is        great, heat dissipation of the surface-emitting semiconductor        element 38 is easy when the surface-emitting semiconductor        element 38 is bonded to the heatsink 43 at the surface of the        GaN substrate 31 as illustrated in FIG. 3. In addition, beam        deformation due to the thermal lens effect is very small in        surface-emitting semiconductor elements. Therefore, the laser        apparatus of FIG. 3 can achieve higher output power than the        conventional laser apparatuses using semiconductor laser        elements.    -   (b) High speed modulation of the output laser light of the laser        apparatus of FIG. 3 can be achieved by directly modulating the        semiconductor laser element 24, while high speed modulation is        difficult in the conventional solid-state laser.    -   (c) Since the semiconductor laser element 24 can be a broad area        type semiconductor laser element as described with reference to        FIG. 1, the semiconductor laser element 24 can emit laser light        with high output power (e.g., 1 to 10 watts). Therefore, the        output power of the laser apparatus of FIG. 3 can be hundreds of        milliwatts to several watts.    -   (d) The surface-emitting semiconductor element 38 is excited        with light, and is therefore different from the usual        semiconductor laser elements driven by current injection, in        that the surface-emitting semiconductor element 38 is free from        the aforementioned problem of deterioration with elapse of time        due to short-circuit currents caused by diffusion of dopants        such as magnesium. Thus, the lifetime of the laser apparatus of        FIG. 3 is long.

Second Embodiment

The semiconductor laser element used as an excitation light source inthe second embodiment emits laser light in the 410 nm band as excitationlight. FIG. 4 is a cross-sectional view of the semiconductor laser. Thesemiconductor laser element used as an excitation light source in thesecond embodiment is produced as follows.

Initially, an n-type Ga_(1-z1)Al_(z1)N/GaN superlattice cladding layer62 (0<z1<1), an n-type or i-type GaN optical waveguide layer 63, anIn_(1-z2) Ga_(z2) N (doped with Si)/In_(1-z3)Ga_(z3)Nmultiple-quantum-well active layer 64 (0<z2<z3<0.5), a p-typeGa_(1-z5)Al_(z5)N carrier blocking layer 65 (0<z5<0.5), an n-type ori-type GaN optical waveguide layer 66, a p-type Ga_(1-z1)Al_(z1)N/GaNsuperlattice cladding layer 67 (0<z1<1), and a p-type GaN contact layer68 are formed on an n-type GaN (0001) substrate 61 by organometallicvapor phase epitaxy. Thereafter, a SiO₂ insulation film 69 is formedover the p-type GaN contact layer 68, and a stripe area of the SiO₂insulation film 69 having a width of about 100 μm is removed by normallithography. Then, a p electrode 70 is formed over the SiO₂ insulationfilm 69 and the stripe area of the p-type GaN contact layer 68, thesubstrate 61 is polished, and an n electrode 71 is formed on thepolished surface. Finally, a resonator is formed by cleavage, and a highreflectance coating and a low reflectance coating are provided on therespective end surfaces so as to form a resonator. Then, theconstruction of FIG. 4 is formed into a chip.

FIG. 5 is a cross-sectional view of a surface-emitting semiconductorelement which is also used in the laser apparatus as the secondembodiment of the present invention. The surface-emitting semiconductorelement of FIG. 5 is excited with excitation laser light emitted fromthe semiconductor laser element of FIG. 4, and oscillates in a singletransverse mode. The surface-emitting semiconductor element used in thesecond embodiment is produced as follows.

Initially, an Al_(x4)Ga_(1-z4)N layer 82 (0<z4<0.5), a GaN opticalconfinement layer 83, an In_(1-z2)Ga₂N/In_(1-z3)Ga_(z3)Nmultiple-quantum-well active layer 84 (0<z2<z3<0.5), a GaN opticalconfinement layer 85, and a superlattice distributed reflection film 86are formed on a GaN (0001) substrate 81 by organometallic vapor phaseepitaxy, where the reflection film 86 is comprised of two pairs of anAlN and GaN layers, the AlN layer in each pair has a thickness ofλ/4n_(AlN), the GaN layer in each pair has a thickness of λ/4n_(GaN), λis an oscillation wavelength of the surface-emitting semiconductorelement of FIG. 5, and n_(AlN) and n_(GaN) are the refractive indexes ofAlN and GaN at the oscillation wavelength λ, respectively. Next, adistributed reflection film 87 is formed over the construction layeredas above, by electron beam evaporation, where the distributed reflectionfilm 87 is comprised of at least one pair of SiO₂ and ZrO₂ layers, theSiO₂ layer in each pair has a thickness of λ/4n_(SiO2), the ZrO₂ layerin each pair has a thickness of λ/4n_(ZrO2), λ is an oscillationwavelength of the surface-emitting semiconductor element of FIG. 5, andn_(SiO2) and n_(ZrO2) are the refractive indexes of SiO₂ and ZrO₂ at theoscillation wavelength λ, respectively. Thereafter, the substrate 81 ispolished, and a ZrO₂ antireflection coating 88 having a thickness ofλ/4_(ZrO2) is provided on the polished surface of the substrate 81.Finally, the layered structure formed as above is cleaved, and furtherformed into a chip.

In order to sufficiently absorb the excitation laser light, it ispreferable that the number of quantum wells in the multiple-quantum-wellactive layer 84 is 20 or more, and it is further preferable that thenumber of the quantum wells is about 24 since the surface-emittingsemiconductor element 89 is prone to crack due to excessive thicknesswhen the number of the quantum wells exceeds 24.

The wavelength λ of light emitted from the semiconductor laser element89 of FIG. 5 can be controlled in the range between 380 and 560 nm byappropriately adjusting the composition of the In_(z3)Ga_(1-z3)Nmultiple-quantum-well active layer 84.

FIG. 6A is a diagram illustrating the construction of the laserapparatus as the second embodiment of the present invention.

The laser apparatus of FIG. 6A comprises the semiconductor laser element74 as an excitation light source, the surface-emitting semiconductorelement 89 bonded to a heatsink 106 at the surface of the distributedreflection film 87, a concave mirror 105 as an output mirror, aresonator 109 formed by the concave surface of the concave mirror 105and a reflection mirror realized by the reflection films 86 and 87 ofthe surface-emitting semiconductor element 89, and a Brewster plate 104arranged in the resonator 109.

In the construction of FIG. 6A, excitation laser light 107 emitted fromthe semiconductor laser element 74 is collected by the lens 102 into thesemiconductor layers of the surface-emitting semiconductor element 89,and excites the surface-emitting semiconductor element 89. Then, lightemitted by the surface-emitting semiconductor element 89 resonates inthe resonator 109, and laser light 108 exits from the output mirror 105.

In the laser apparatus of FIG. 6A, the surface-emitting semiconductorelement 89 is bonded to the heatsink 106 at the surface of thedistributed reflection film 87, which is an end surface of thesurface-emitting semiconductor element which is near to the activelayer. Therefore, heat generated in the active layer can be easilydissipated into the heatsink 106, and thus the laser apparatus of FIG.6A can emit laser light in a stable oscillation mode.

Alternatively, the incident direction of the excitation laser light 107from the semiconductor laser element 74 may be inclined as illustratedin FIG. 6B so as to suppress light returned from the resonator 109 tothe semiconductor laser element 74.

Third Embodiment

FIG. 7 is a cross-sectional view of a surface-emitting semiconductorelement which is used in the laser apparatus as the third embodiment ofthe present invention. The surface-emitting semiconductor element ofFIG. 7 is excited with excitation laser light emitted from thesemiconductor laser element of FIG. 4, and oscillates in a singletransverse mode. The surface-emitting semiconductor element used in thethird embodiment is produced as follows.

Initially, a superlattice distributed reflection film 112, a GaN opticalconfinement layer 113, an In_(1-z2)Ga_(z2)N/In_(1-z3)Ga_(z3)Nmultiple-quantum-well active layer 114 (0<z2<z3<0.5), a GaN opticalconfinement layer 115, an Al_(z4)Ga_(1-z4)N carrier confinement layer116 (0<z4<0.5), and a ZrO₂ layer 117 are formed on a GaN (0001)substrate 111 by organometallic vapor phase epitaxy. The superlatticedistributed reflection film 112 is comprised of two pairs of an AlN andGaN layers, the AlN layer in each pair has a thickness of λ/4n_(AlN),the GaN layer in each pair has a thickness of λ/4n_(GaN)λ is anoscillation wavelength of the surface-emitting semiconductor element ofFIG. 7, and n_(AlN) and n_(GaN) are the refractive indexes of GaN andAlN at the oscillation wavelength λ, respectively. In addition, the ZrO₂layer 117 has a thickness of λ/4n_(ZrO2), where n_(ZrO2) is therefractive index of ZrO₂ at the oscillation wavelength λ. Next, thesubstrate 111 is polished, and a ZrO₂ antireflection coating 118 havinga thickness of λ/4n_(ZrO2) is formed on the polished surface of thesubstrate 111. Thereafter, the layered structure formed as above iscleaved, and further formed into a chip.

In order to sufficiently absorb the excitation laser light, it ispreferable that the number of quantum wells in the multiple-quantum-wellactive layer 114 is 20 or more, and it is further preferable that thenumber of the quantum wells is about 24 since the surface-emittingsemiconductor element 118 is prone to crack due to excessive thicknesswhen the number of the quantum wells exceeds 24.

The constructions of two examples of laser apparatuses as the thirdembodiment are illustrated in FIGS. 8A and 8B. In the constructions ofFIGS. 8A and 8B, the surface-emitting semiconductor element 118 of FIG.7 is excited with excitation laser light emitted from the semiconductorlaser element 74, which is illustrated in FIG. 5. The constructions ofFIGS. 8A and 8B are respectively identical with the constructions ofFIGS. 6A and 6B, except that the surface-emitting semiconductor element119 has the construction of FIG. 7, and the surface-emittingsemiconductor element 119 is bonded to the heatsink 106 at the surfaceof the GaN substrate 111.

Since the GaN substrate 111 is transparent to the excitation laser light107, it is possible to excite the surface-emitting semiconductor element119 through the GaN substrate 111. Alternatively, when a sapphiresubstrate is used, instead of the GaN substrate, excitation laser lightcan also be supplied to the surface-emitting semiconductor elementthrough the sapphire substrate since the sapphire substrate is alsotransparent to the excitation laser light.

In addition, since the thermal conductivity of the GaN substrate isgreat, heat generated in the surface-emitting semiconductor element canbe easily dissipated into the heatsink when the surface-emittingsemiconductor element is bonded to the heatsink as illustrated in FIG.8A or 8B. Further, beam deformation due to the thermal lens effect orthe like is very small.

Additional Matters

(i) One or more wavelength selection elements such as Lyot filters oretalons may be further arranged in the resonator in each of the first tothird embodiments so as to realize oscillation in a single longitudinalmode.

(ii) The active layer of the surface-emitting semiconductor element ineach embodiment may be made of a GaNAs or InGaNAs material, instead ofInGaN materials, so as to enable oscillation at a longer wavelength.

(iii) The semiconductor laser elements for emitting excitation laserlight in the first to third embodiments are not limited to thebroad-area type, and may be α-DFB (distributed feedback) semiconductorlasers, MOPA (master oscillator power amplifier) semiconductor lasers,or other normal semiconductor lasers. In particular, the MOPAsemiconductor lasers, which have a tapered structure, enablehigh-density light collection.

(iv) The laser apparatuses according to the present invention canoperate not only in a continuous wave (CW) mode, but also in aQ-switched mode.

(v) Since it is easy to obtain high peak power from the InGaNsemiconductor laser elements, and the excitation light source in each ofthe first to third embodiments is realized by the InGaN semiconductorlaser element, it is also easy to obtain pulsed light having high peaksby driving the InGaN semiconductor laser element in a pulse mode, andexciting the surface-emitting semiconductor element with the InGaNsemiconductor laser element.

(vi) In addition, all of the contents of Japanese Patent Application No.11(1999)-257529 are incorporated into this specification by reference.

1.-9. (canceled)
 10. A laser apparatus comprising: a semiconductor laserelement having a first active layer made of a GaN-based compound, andemitting first laser light; a surface-emitting semiconductor elementhaving a second active layer made of a GaN-based compound, being excitedwith said first laser light, and emitting second laser light, and atleast one third semiconductor laser element, each having a third activelayer made of a GaN-based compound, and emits third laser light, saidsurface-emitting semiconductor element being excited with said thirdlaser light together with said first laser light.
 11. A laser apparatusaccording to claim 10, wherein said second active layer includes aplurality of quantum wells.
 12. A laser apparatus according to claim 11,wherein said second active layer includes twenty or more quantum wells.13. A laser apparatus according to claim 10, wherein said first activelayer is made of an InGaN or GaN material, and said second active layeris made of an InGaN material.
 14. A laser apparatus according to claim13, wherein said second active layer includes a plurality of quantumwells.
 15. A laser apparatus according to claim 14, wherein said secondactive layer includes twenty or more quantum wells.
 16. A laserapparatus according to claim 10, wherein said first active layer is madeof an InGaN or GaN material, and said second active layer is made of aGaNAs or InGaNAs material.
 17. A laser apparatus according to claim 16,wherein said second active layer includes a plurality of quantum wells.18. A laser apparatus according to claim 17, wherein said second activelayer includes twenty or more quantum wells.
 19. A laser apparatuscomprising: a semiconductor laser element having a first active layermade of a GaN-based compound, and emitting first laser light; asurface-emitting semiconductor element having a second active layer madeof a GaN-based compound, being excited with said first laser light, andemitting second laser light, and at least one third semiconductor laserelement, each having a third active layer made of a GaN-based compound,and emits third laser light, said surface-emitting semiconductor elementbeing excited with fourth laser light which is produced by polarizationcoupling of said first and third laser light.
 20. A laser apparatusaccording to claim 19, wherein said second active layer includes aplurality of quantum wells.
 21. A laser apparatus according to claim 20,wherein said second active layer includes twenty or more quantum wells.22. A laser apparatus according to claim 19, wherein said first activelayer is made of an InGaN or GaN material, and said second active layeris made of an InGaN material.
 23. A laser apparatus according to claim22, wherein said second active layer includes a plurality of quantumwells.
 24. A laser apparatus according to claim 23, wherein said secondactive layer includes twenty or more quantum wells.
 25. A laserapparatus according to claim 19, wherein said first active layer is madeof an InGaN or GaN material, and said second active layer is made of aGaNAs or InGaNAs material.
 26. A laser apparatus according to claim 25,wherein said second active layer includes a plurality of quantum wells.27. A laser apparatus according to claim 26, wherein said second activelayer includes twenty or more quantum wells. 28.-36. (canceled)
 37. Alaser apparatus comprising: a semiconductor laser element having a firstactive layer made of a GaN-based compound, and emitting first laserlight; a surface-emitting semiconductor element being excited with saidfirst laser light, emits second laser light, and having a second activelayer made of a GaN-based compound and a first mirror arranged on oneside of said second active layer; a second mirror arranged outside ofsaid surface-emitting semiconductor element so that said first andsecond mirrors form a resonator; and at least one third semiconductorlaser element, each having a third active layer made of a GaN-basedcompound, and emits third laser light, said surface-emittingsemiconductor element being excited with said third laser light togetherwith said first laser light.
 38. A laser apparatus according to claim37, wherein said second active layer includes a plurality of quantumwells.
 39. A laser apparatus according to claim 38, wherein said secondactive layer includes twenty or more quantum wells.
 40. A laserapparatus according to claim 37, wherein said first active layer is madeof an InGaN or GaN material, and said second active layer is made of anInGaN material.
 41. A laser apparatus according to claim 40, whereinsaid second active layer includes a plurality of quantum wells.
 42. Alaser apparatus according to claim 41, wherein said second active layerincludes twenty or more quantum wells.
 43. A laser apparatus accordingto claim 37, wherein said first active layer is made of an InGaN or GaNmaterial, and said second active layer is made of a GaNAs or InGaNAsmaterial.
 44. A laser apparatus according to claim 43, wherein saidsecond active layer includes a plurality of quantum wells.
 45. A laserapparatus according to claim 44, wherein said second active layerincludes twenty or more quantum wells.
 46. A laser apparatus comprising:a semiconductor laser element having a first active layer made of aGaN-based compound, and emitting first laser light; a surface-emittingsemiconductor element being excited with said first laser light, emitssecond laser light and having a second active layer made of a GaN-basedcompound and a first mirror arranged on one side of said second activelayer; a second mirror arranged outside of said surface-emittingsemiconductor element so that said first and second mirrors form aresonator; and at least one third semiconductor laser element, eachhaving a third active layer made of a GaN-based compound, and emitsthird laser light, said surface-emitting semiconductor element beingexcited with fourth laser light which is produced by polarizationcoupling of said first and third laser light.
 47. A laser apparatusaccording to claim 46, wherein said second active layer includes aplurality of quantum wells.
 48. A laser apparatus according to claim 47,wherein said second active layer includes twenty or more quantum wells.49. A laser apparatus according to claim 46, wherein said first activelayer is made of an InGaN or GaN material, and said second active layeris made of an InGaN material.
 50. A laser apparatus according to claim49, wherein said second active layer includes a plurality of quantumwells.
 51. A laser apparatus according to claim 50, wherein said secondactive layer includes twenty or more quantum wells.
 52. A laserapparatus according to claim 46, wherein said first active layer is madeof an InGaN or GaN material, and said second active layer is made of aGaNAs or InGaNAs material.
 53. A laser apparatus according to claim 52,wherein said second active layer includes a plurality of quantum wells.54. A laser apparatus according to claim 53, wherein said second activelayer includes twenty or more quantum wells.